Fiber reinforced plastic pipe has come into reasonably extensive use in recent years for handling corrosive materials, petrochemicals, and the like where metallic pipe is unsuitable. Glass fiber reinforcements are employed so that pipe can withstand appreciable pressures. Epoxy resins, commonly with a thin lining resistant to chemicals are often used. The pipes are formed by winding rovings of glass fiber coated with epoxy resin in helical paths around a cylindrical mandrel and curing the resin. Such pipes can be made economically and it is desirable to make economical fittings for such pipes, such as tees and elbows.
Techniques have been developed for economically winding pipe elbows which are essentially sharply curved sections of pipe having two ends. Economical techniques have not been developed for winding pipe tees since they have a much more complicated geometry. Unlike an elbow with two ends, a pipe tee has three ends. This greatly complicates the winding problems since it is important to cover all areas of the tee with a sufficient thickness of fibers with proper orientation for resisting the complex stress distributions in a tee without excess thickness being built up in other areas.
The patterns used for winding the tee must keep the rovings in contact with or very close to the mandrel on which the tee is wound for maintaining a desired internal geometry in the tee. "Bridging" of fibers across a concave portion of the tee can result in very low strength in the bridged areas and require excessive quantities of reinforcement for resisting operating pressures. Commercially available tees are made by hand by laying up a montage of strips of woven fabric. Such assembly techniques are costly since the woven glass fabric is expensive and a large amount of hand labor is required. Quality control with such assembly procedures is also difficult and costly.
There is, therefore, a substantial need for an economical technique for winding fiber reinforced plastic pipe tees, preferably using relatively inexpensive glass roving instead of costly woven fabric. Such a technique must cover all areas of the tee with an adequate thickness of material to resist the complex stress distribution in a pipe tee without excessive waste or thickness in some areas of the tee.
One area of particular concern because of poor stress distribution is known as the diaphragm and comprises a roughly triangular area on each side of the tee near where the three arms of the tee intersect. This area tends to be relatively flat and subject to biaxial stresses which are large and hard to resist in a non-ductile material such as fiber reinforced plastic.
A variety of configurations have been proposed or used for metal tees, or valve bodies which can be considered to be tees with internal flow control mechanisms. The configuration of the body of a valve is commonly dictated by the internal structure rather than controlled by the stress distribution. The configuration of metal tees and valves is not apropos to fiber reinforced plastic pipe tees because of the inherent ductility of the metal as contrasted with brittle behavior of the fiber reinforced plastic. Because of such ductility stress distributions can be tolerated which are unacceptable in a fiber reinforced plastic pipe tee. Such metal bodies, of course, have no problems associated with filament winding.
A variety of configurations have been proposed for alleviating the adverse stress distributions in the diaphragm area, such as, for example, making the center portion of the tee as a sphere or ellipse. Such a tee is in the form of a sphere with three arms protruding from it. Such a pipe tee is described and illustrated in U. S. Pat. No. 3,765,979, by Thomas. The pattern of windings provided in that patent can produce a tee suitable for resisting internal pressure but unsuited for a rigid piping system.
The tee in the Thomas patent is suitable for a "blocked" piping system where the pipes and/or fittings are rigidly mounted or "blocked" so that no appreciable longitudinal loads are transmitted through the joints between pipes and fittings. In such a system an O-ring joint or the like is used between the end of a piece of pipe and the tee, for example. Such a joint can accommodate limited longitudinal motion, hence imposes little, if any, longitudinal stress on the tee. The blocking of the piping system also minimizes bending loads. In such a system the pipe tee is subject to internal pressure stresses and is virtually free of longitudinal stress and bending.
Since construction of a blocked piping system with rigid mounting is costly, it is considerably more common to form an integral pipe system by cementing the pipe fittings, including tees, onto the pipe. In such a system the pipe tee is essentially a closed end pressure vessel which is subject to longitudinal stress as well as the hoop stresses due to internal pressure. The pipe tee is also quite likely to be subject to bending stresses due to installation misalignments or changes in dimension during operation. A pipe tee suitable for such service requires strength in directions not provided by the winding pattern in the Thomas patent, since that tee is designed for a different type of service. Further, since the tee in the Thomas patent has limited longitudinal and bending stresses an abrupt transition between the central body portion and the three tubular extensions can be acceptable. A different configuration must be provided for a tee employed in a system having substantial longitudinal and bending stresses both for resisting such stresses and to permit winding filaments in the necessary directions without bridging of the filaments across recessed regions of a mandrel. Winding patterns must be developed which cover such a tee thoroughly with filaments extending in the principal stress directions without use of excessive material and without bridging. It is also desirable that the winding patterns be readily implemented in mechanized winding equipment so that tees can be made economically and with good reproducibility.
It is also desirable to develop a configuration for the diaphragm area of a tee which provides good resistance to biaxial stresses and avoids problems in winding the tee.